Electroless plating refers to the autocatalytic or chemical reduction of aqueous metal ions plated on a base substrate. Deposits made by electroless plating have unique metallurgical characteristics. The coatings may have good uniformity, excellent corrosion resistance, wear and abrasion resistance, nonmagnetic and magnetic properties, solderability, high hardness, excellent adhesion, and low coefficient of friction. The deposits can be made onto a wide range of substrates, including both metallic and nonmetallic surfaces.
Electroless bath compositions typically contain an aqueous solution of metal ions to be deposited, catalysts, one or more reducing agents, one or more complexing agents and bath stabilizers, all of which are tailored to specific metal ion concentration, temperature and pH range. In electroless metal depositing, use is made of a chemical reducing agent, thus avoiding the need to employ an electrical current as required in conventional electroplating operations.
In an electroless plating process, metal ions are reduced to metal through the action of chemical reducing agents serving as electron donors. The metal ions are electronic acceptors, which react with the electron donors to form a metal that becomes deposited on the substrate. The catalyst is simply the surface provided to the bath, which serves to accelerate the electroless chemical reaction to allow oxidation and reduction of the metal ion to metal.
One of the most common electroless plating operations involves the electroless deposition of nickel or a nickel alloy. A plating bath of this type generally comprises at least four ingredients, namely, a source of nickel ions, a hypophosphite compound as a reducing agent, an acid or hydroxide pH adjusting compound, and a complexing agent for the metal ions to prevent their premature precipitation.
The uniform dispersion of micron or sub-micron particles in an electroless metal deposit, such as nickel, can enhance the wear, abrasion resistance and/or lubricity of the deposit over base substrates and conventional electroless deposits. Composites containing fluoropolymers, such as polytetrafluoroethylene (PTFE), natural and synthetic (polycrystalline) diamonds, ceramics, chromium carbide, silicon carbide, and aluminum oxide have been co-deposited in formulations of the prior art.
One commonly used composite material in an electroless nickel plating bath is PTFE. In order to enable dispersions of PTFE particles (or other codeposited materials) to perform adequately in the plating bath, surface active agents must be added to the bath. However, the addition of certain surface active agents can lead to the formation of electroless composite films having an irregular pattern.
U.S. Pat. No. 6,273,943 to Chiba et al., the subject matter of which is herein incorporated by reference in its entirety, discusses that electroless composite plating solutions are significantly shorter in life than electroless plating solutions containing no composite material such as PTFE powder. Another problem noted by Chiba et al. is that the resultant composite film has satin-like or lusterless appearance, thus leading to the likelihood of the surface being roughened and the occurrence of various types of appearance defects. Chiba et al. also note that in the use of electroless composite plating solutions, the deposition rate is slow and that the plating solution is likely to decompose.
Various methods have been suggested to ensure the formation of composite films of uniform appearance having a high content of particles codeposited. For example, U.S. Pat. No. 5,232,744 to Nakamura et al., the subject matter of which is herein incorporated by reference in its entirety, describes an electroless plating bath for the electroless deposition of a composite film consisting essentially of a metal matrix and water-insoluble particles or fibers dispersed therein comprising an amine or ammonium salt added to the electroless plating bath to enable the formation of a composite film of good uniform appearance having an increased content of particle or fibers codeposited therein.
As is readily seen, there is a need for an improved electroless composite plating solution that has good performance, a good deposition rate and uniformity of the resultant film. In addition there is a need for a plating solution that has a long life, is low in cost and easy to handle, and that is stable in performance together with a good appearance, even after long-term use of the plating bath. In particular, there is a need for a stable PTFE dispersion that continues to work as the plating bath ages and that can produce an electroless nickel deposit with greater than 20 percent by volume of PTFE in the deposit.
To that end, the inventors have discovered that the use of a low viscosity silicone glycol surfactant in PTFE dispersions added to an electroless nickel plating bath can provide an improved composite coating on the surface of a substrate.